Closed loop active cancellation technique (ACT) - based RF power amplifier linearization architecture

ABSTRACT

A closed loop, active cancellation technique (ACT)-based RF power amplifier linearization architecture injects a pilot tone as a ‘pseudo distortion’ signal into signal paths through first and second matched RF amplifiers, and a set of power minimization loops are closed around the RF amplifier pair. The power minimizing control loops control a set of vector modulators such that both the injected pilot tone and intermodulation distortion products are canceled, while RF carrier components constructively sum in the composite output of the two RF amplifiers.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims the benefit of now-abandoned U.S.Provisional Patent Application Serial No. 60/259,012, filed Dec. 29,2000, by S. Avis et al, entitled: “Improved ACT Pre-DistortionLinearized Power Amplifier,” assigned to the assignee of the presentapplication and the disclosure of which is incorporated herein.

FIELD OF THE INVENTION

[0002] The present invention relates in general to communicationsystems, and is particularly directed to a new and improved, closedloop, Active Cancellation Technique (ACT)-based RF power amplifierlinearization architecture having parallel RF amplifiers coupled inintermod-complementing predistortion paths of the type disclosed in theU.S. patent to Mucinieks, U.S. Pat. No. 6,111,462, (hereinafter referredto as the '462 patent and the disclosure of which is incorporatedherein). By injecting a pilot tone into the signal transport paths ofeach of the RF power amplifiers, the invention is able to close a set ofvector modulation control loops and thereby track and cancelintermodulation distortion products from the composite output of thepair of RF amplifiers.

BACKGROUND OF THE INVENTION

[0003] Communication service providers are subject to very strictbandwidth usage spectrum constraints, including technically mandatedspecifications and regulations imposed by the Federal CommunicationsCommission (FCC). These rules require that sideband spillage, namely theamount of energy spillover outside a licensed band of interest, besharply attenuated (e.g., on the order of 50 dB). Although theseregulations may be easily met for traditional forms of modulation, suchas FM, they are difficult to achieve using more contemporary, digitallybased modulation formats, such as M-ary modulation.

[0004] Attenuating the sidebands sufficiently to meet industry andregulatory-based requirements by such modulation techniques requiresvery linear signal processing systems and components. Although linearcomponents can be produced at a reasonable cost at the relatively narrowbandwidths (baseband) of telephone networks, linearizing inherentlynon-linear components such as RF power amplifiers can be prohibitivelyexpensive.

[0005] A fundamental difficulty in linearizing RF power amplifiers isthe fact that they generate unwanted intermodulation distortion products(IMDs) which manifest themselves as spurious signals in the amplified RFoutput signal, such as spectral regrowth or spreading of a compactspectrum into spectral regions that do not appear in the RF inputsignal. This distortion causes the phase/amplitude of the amplifiedoutput signal to depart from the phase/amplitude of the input signal,and may be considered as an incidental (and undesired) amplifier-sourcedmodulation of the RF input signal.

[0006] A brute force and relatively inefficient approach to linearize anRF power amplifier is to build the RF amplifier as a large, high powerdevice, and then operate the amplifier at a very low power level(namely, at only a small percentage of its rated output power), wherethe RF amplifier's transfer characteristic is substantially linear. Anobvious drawback to this approach is the overkill penalty—a costly,inefficient and large sized RF device.

[0007] Other prior art linearization techniques include baseband polar(or Cartesian) feedback, post-amplification, feed-forward correction,and pre-amplification, pre-distortion correction. In the first approach,the output of the RF power amplifier is compared to the input, and abaseband error signal is used to directly modulate the signal whichenters the amplifier. In the second approach, error (distortion)

[0008] present in the RF amplifier's output signal is extracted,amplified to the proper level, and then reinjected (as a complement ofthe error signal back) into the output path of the amplifier, so that(ideally) the RF amplifier's distortion is effectively canceled.

[0009] Pursuant to a third approach, a predistortion signal is injectedinto the RF input signal path upstream of the RF amplifier. Ideally, thepredistortion signal has a characteristic equal and opposite to thedistortion expected at the output of the RF amplifier. As a result, whensubjected to the (distorting) transfer characteristic of the RFamplifier, it effectively cancels the distortion in the output.Predistortion may be made adaptive by measuring the distortion at theoutput of the RF amplifier and adjusting the predistortion controlsignal to minimize the distortion of the output signal of the poweramplifier during real time operation.

[0010] In accordance with the ‘Active Cancellation Technique’ (ACT) RFpower amplifier linearization scheme described in the '462 Patent, andshown diagrammatically in FIG. 1, high efficiency RF power amplifierlinearization is achieved by an open loop technique that adjusts signalcomponents driving a pair of effectively ‘matched’ RF power amplifiersA₁ and A₂, such that one RF power amplifier ‘pre-distorts’ the other.Being matched implies that the two amplifiers A₁, A₂ have essentiallythe same transfer characteristics—both in terms of their intended RFperformance and unwanted IMD components they inherently introduce intotheir amplified outputs.

[0011] More particularly, an RF input signal to be amplified is split bya directional coupler CPL1 into two paths. A first path includes anattenuator or scaling pad ATT and a controlled gain adjustment G1 and aphase adjustment element Φ1, which adjust the amplitude and phase of theRF input signal prior to being amplified by the main amplifier A₁. Theoutput of the main path amplifier A₁ is coupled through a delay stageDL2 to a first input of an output combining stage OCS (such as aquadrature hybrid).

[0012] A second split RF input signal path is used to derive a signalcontaining both the original RF input signal to be amplified by thesecond ‘matched’ amplifier A₂, and a complementary version of the IMDproducts which each of the two amplifiers inherently introduces. IMDproducts are extracted using carrier cancellation circuitry WS1-WC1similar to that found in most conventional feed-forward RF poweramplifiers. The extracted distortion products are adjusted in amplitudeand phase by gain and phase control elements G1 and Φ1 and combined withan appropriately delayed and scaled sample of the RF input signal atWC2.

[0013] For this purpose, the second path from the directional couplerCPL1 is coupled through a delay stage DL1 to a first input of(Wilkinson) splitter WS1, a first output of which is coupled to(Wilkinson) combiner WC1. A second output of splitter WS1 is coupledthrough a variable gain stage G2 to a first input of further (Wilkinson)combiner WC2, a second input of which is coupled to the output of thecombiner WC1. A second input of combiner WC1 is coupled to a directionalcoupler CPL2 installed in the output path of main path amplifier A₁.

[0014] The output of combiner WC2, which is a composite of the RF inputsignal and complementary distortion products extracted from the RFamplifier A₁, is coupled through a variable gain stage G3 and variablephase adjustor Φ2 to the matched RF amplifier A₂. The output of RFamplifier A₂ is coupled to a second input of output combining stage OCS.

[0015] The amplitude of the RF input signal component of the compositeRF signal driving the amplifier A₂ is adjusted to be the same as theamplitude of the pure RF input signal driving amplifier A₁. Namely, thephase and amplitude of the distortion products are adjusted so that theynot only cancel the distortion products generated by the input signalsapplied to the error amplifier A₂, but also replace these distortionproducts with equal amplitude anti-phase replicas of these products.Thus, the delayed output of amplifier A₁ and the undelayed output of theamplifier A₂ contain equal phase and amplitude amplified RF inputsignals and equal amplitude anti-phase distortion products. Thus,distortion components resulting from the RF input signal componentsdriving both amplifiers are essentially the same.

[0016] In the output combining stage OCS, these signals are summed, sothat (desired) amplified RF (carrier) signals add and (unwanted)distortion products cancel. The output from the combining stage OCS istherefore an amplified version of the RF input signal, that issubstantially free of distortion, even though both amplifiers containdistortion products at their outputs. Both amplifiers contributeessentially equal amounts of amplification power to the output of theoverall system. Operating efficiency is better than that of aconventional feed forward amplifier because essentially the entirety ofboth amplifiers' output power appears at the output of the combiningstage.

[0017] It should be noted that the ACT architecture of FIG. 1 is not aclassic feed-forward architecture. Rather, it is a very effective typeof dual amplifier-based, RF pre-distortion amplifier structure, in whichthe source of the energy used to pre-distort the matched amplifier A₂ isproduced by an identical (main) amplifier A₁, driven by essentially thesame input signals as its matched counterpart. The level of distortioncomponents in the energy driving the matched amplifier A₂ is on theorder of 30 dB below the RF input signal component. Thus, the dynamicsof both amplifiers is controlled by the dominant input signal energy.

[0018] Now although the ACT amplifier-based linearization schemedescribed in the '462 Patent is very effective for achieving a level ofnon-linear distortion correction at least on the order of 20 dB andgreater, a given production device may not be capable of maintainingthis level of performance over a wide range of ambient temperature andvarying power supply voltage.

[0019] One of the reasons for this potential performance shortcoming ofthe linearization scheme of the '462 patent is the fact that it is anopen loop architecture, and operates on the assumption that since theattenuators which adjust the power to the ‘matched’ main and poweramplifiers are slaved together, it can be reasonably inferred that theresulting output signal and distortion energy delivered by eachamplifier will be exactly the same. However, investigation by thepresent inventors on substantial numbers of practical production poweramplifiers linearized in accordance with the '462 Patent approach hasshown this not to be the case.

SUMMARY OF THE INVENTION

[0020] Pursuant to a first embodiment of the invention, this amplifieroutput signal and distortion energy inequality problem is effectivelyremedied by injecting a pilot tone as a ‘pseudo distortion’ signal intothe signal transport paths of each of the pair of RF power amplifiers,in order to track intermodulation distortion products produced by eachamplifier. Prescribed signal transport paths of the dual amplifierarchitecture are monitored by a set of minimization control loops, whichcontrol associated vector modulators, such that both the injected pilottones and intermodulation distortion products are canceled, while RFcarrier components are mutually reinforced or constructively sum in thecomposite output of the pair of RF amplifiers.

[0021] To this end, the signal flow path to one of the two ‘matched’ RFamplifiers includes a first vector modulator, which is controlled by afirst digital signal processor-executed carrier power control mechanism.This first carrier power control mechanism monitors carrier powermeasured by a detector at the output of the carrier cancellation loop,and adjusts the operation of the first vector modulator, so as toeffectively minimize carrier energy, leaving only an injected pilot toneand amplifier distortion energy at the output of the carriercancellation loop.

[0022] A relatively low level, out-of-band pilot tone, which serves as a‘pseudo distortion’ signal, is injected into the signal transport pathsof the RF amplifiers, and is used to track and cancel intermodulationdistortion products produced by each amplifier. The amplifier outputsthus contain a pilot tone component in addition to the desired RF signaland undesired IMDs. Since neither IMDs nor the pilot tone are part ofthe desired modulated carrier signal being amplified, they constituteunwanted distortion. The use of the pilot tone as a ‘pseudo distortion’,signal allows the pilot to be treated as representative of whatevernoise or distortion is produced by the amplifier pair. By minimizing thecontribution of the pilot tone to the composite output signal producedby the amplifier pair, IMDs are also minimized.

[0023] A further vector modulator is also installed in the first signaltransport path feeding the second amplifier, and is controlled inaccordance with a monitored pilot tone-based control loop, so that theinjected pilot tone (and therefore any intermodulation distortionproducts) cancel, at the composite output of the two RF amplifiers.

[0024] A second vector modulator is further installed in the secondsignal transport path feeding the second amplifier, and is controlled inaccordance with a power detector coupled to a terminated port of anoutput combiner. When the desired RF carrier components at theterminated port of the output combiner null, the carrier power levels atfirst and second input ports of the output combiner are essentiallyequal and sum constructively at the output signal port of the combiner.

[0025] The second vector modulator adjusts a component of input RFcarrier energy so as to ensure that the RF carrier energy applied to thesecond amplifier is the same as that applied to the one amplifier. Forthis purpose, the output of the power detector monitoring the terminatedoutput of the output combiner is applied to a second carrier powerminimization-based control mechanism within the digital signalprocessor. This second carrier power control mechanism controls thesecond vector modulator so as to minimize any carrier leakage energy atthe output combiner's terminated port, and thereby equalize the carrierinputs to the two amplifiers.

[0026] The amplitude and phase of the pilot energy injected into thesecond amplifier is adjusted by a gain/phase adjustor, so that whendistortion is minimized at the output of the composite amplifier, thepilot energy is also minimized. The vector modulator installed in thefirst signal transport path is controlled so as to minimize both pilotenergy and distortion at the signal output port of the output combiner.This ensures that the total contribution of the pilot tone componentfrom the output of the first amplifier is exactly the opposite of thepilot tone component in the output of the second amplifier.

[0027] In accordance with a second embodiment, a feed-forward loop iswrapped around the closed loop, pre-distortion architecture of the firstembodiment. The addition of the feed forward stage enables theintegrated amplifier architecture containing the RF amplifierlinearization stage and the feed forward stage to deliver extremely highlinearity, including the ability to routinely achievecarrier-to-distortion ratios as high as 85 dB. Efficiency issignificantly better than that which is typically seen in competitivedual loop feed forward power amplifiers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 diagrammatically illustrates an ACT-stage based RF poweramplifier linearization architecture of the type disclosed in theabove-referenced '462 Patent;

[0029]FIG. 2 diagrammatically illustrates a closed loop ACT stage-basedRF power amplifier linearization mechanism in accordance with a firstembodiment of the present invention; and

[0030]FIG. 3 diagrammatically illustrates a second embodiment of thepresent invention in which a feed forward loop is wrapped around theenhanced ACT-based RF power amplifier linearization architecture of FIG.2.

DETAILED DESCRIPTION

[0031] Before detailing the closed loop ACT stage-based RF poweramplifier linearization architecture of the present invention, it shouldbe observed that the invention resides primarily in an arrangement ofconventional RF communication circuits and associated digital signalprocessing components and attendant supervisory control circuitry, thatcontrols the operations of such circuits and components. Consequently,the configuration of these circuits and components and the manner inwhich they are interfaced with other communication system equipmenthave, for the most part, been illustrated in the drawings by readilyunderstandable block diagrams, which show only those specific detailsthat are pertinent to the present invention, so as not to obscure thedisclosure with details which will be readily apparent to those skilledin the art having the benefit of the description herein.

[0032] Thus, the block diagram illustrations are primarily intended toshow its major components in a convenient functional grouping, wherebythe present invention may be more readily understood. In addition, tofacilitate an understanding of signal processing flow through therespective paths of the linearization system of the various embodimentsof the invention, reduced complexity spectral diagrams of the desiredmodulated RF carrier and the unwanted IMD components have been placedthroughout the Figures in association with respective transfer functionsof the system components.

[0033] Referring now to FIG. 2, a first embodiment of a closed loop ACTstage-based RF power amplifier linearization mechanism of the presentinvention is diagrammatically illustrated as comprising an inputterminal 10 to which an RF input signal RF_(IN) (such as a multicarrierRF signal pictorially represented by a carrier frequency pair 12) isapplied. The RF input terminal 10 is coupled through a preamplifier 13to a directional coupler (CPLR 1) 14 installed in a main signal flowpath (designated by arrow MP) of a first, main RF amplifier 20.

[0034] The main signal flow path MP contains a first vector modulator 16cascaded with a buffer amplifier 19 feeding the main RF amplifier 20. Asa non-limiting example, the vector modulator unit 16 may be implementedas a variable gain (attenuator) stage 17 series coupled with a variablephase shifting stage 18. The variable gain stage 17 and phase shiftingstage 18 are controlled by control signals supplied over a control link15 from a first carrier power control mechanism 116 executed within adigital signal processor (DSP) 100.

[0035] As will be described, via an input 101, this first carrier powercontrol mechanism 116 monitors the output of a power detector 37, whichis coupled to a directional coupler 35 in the output path of a (pilottone and distortion signal extracting) carrier cancellation (Wilkinson)combiner 30 of a carrier cancellation loop (loop 1). The carrier powercontrol mechanism 116 executes a standard error minimization (e.g.,power or least mean squared minimization), and adjusts the vectormodulator 16 input to the RF amplifier 20, so that its contribution atthe first input 31 of the carrier cancellation combiner 30 is the sameamplitude and opposite phase as that of the RF carrier at the secondinput of the combiner 30. This effectively minimizes carrier energy,leaving only an injected pilot tone (injected via a directional coupler22 at the input of amplifier 20) and amplifier distortion energy at thecarrier cancellation combiner output 33.

[0036] The directional coupler 22 in the input path MP to main RF poweramplifier 20 is coupled to one output of a (Wilkinson) splitter 23,which is coupled to receive a first, relatively low level, out-of-bandpilot tone generated by a pilot tone generator 24. This low level pilottone is injected into the signal transport paths of each of the RF poweramplifier 20 and a matched companion RF power amplifier 80, and is usedto track and cancel intermodulation distortion products produced by eachamplifier. The vector modulator 25 is used to adjust the amplitude andphase of pilot energy incident upon the RF power amplifier 80, so thatwhen vector modulator 50 is adjusted for optimum distortion cancellationat an output 93 of an output combiner 90, the pilot energy issimultaneously nulled at output 93 as well.

[0037] Because the pilot tone is injected immediately upstream of eachRF amplifier 20/80, each amplifier output will contain a pilot tonecomponent in addition to the desired RF signal and undesired IMDs. Sinceneither IMDs nor the pilot tone are part of the desired modulatedcarrier signal being amplified, they constitute unwanted distortion. Theuse of the pilot tone as a ‘pseudo noise’ signal allows the pilot to betreated as representative of whatever noise or distortion is produced bythe amplifier pair. Therefore, by minimizing the contribution of thepilot tone to the composite output signal produced by the amplifierpair, IMDs are also minimized. As pointed out above, the distortionminimization scheme of the invention controls vector modulators forsignal paths feeding the matched amplifier 80 by a pair of controlloops, one monitoring pilot energy at the output of the composite ACTamplifier, and the other acting to equalize the desired carrier power atthe outputs of the RF power amplifiers 20 and 80, so that the twoinjected pilot tones (and therefore any intermodulation distortionproducts) cancel, while the desired RF carrier components constructivelysum in the composite output of RF amplifiers 20/80.

[0038] A second output of splitter 23 is applied to an amplitude/phaseadjustor 25, which sets the amplitude of the pilot tone and adjusts itsphase, for application via a directional coupler 65 to the input signalpath to RF amplifier 80. The amplitude adjustment is set such that thepilot signal produced at the output of amplifier 80 and applied to aninput 92 of an output combiner 90, shown configured as a four portcombiner hybrid, will have exactly the same amplitude as the pilotsignal produced at the output of amplifier 20 and applied to input 91 ofoutput combiner 90. Since these two injected pilot signals are mutuallyantiphase (owing to the proper phase adjustment imparted byamplitude/phase adjustor 25), for equal amplitude pilot components atthe output of each amplifier, the two pilots subtract from one anotherand are canceled at an output 93 of output combiner 90.

[0039] Because the input path to amplifier 80 also includes a componentof the pilot tone that has been injected into the other amplifier 20 andextracted via the carrier cancellation combiner 30, the combiner'soutput port 93 is monitored via a directional coupler 95 feeding aninput port 103 of a pilot tone receiver, which is coupled to an energyminimization mechanism 110 in the DSP 100. The pilot energy minimizationmechanism controls the vector modulator 50, so as to minimize (drive tozero) any pilot tone energy at the output combiner's output port 93.This serves to ensure that the total contribution of the pilot tonecomponent from the output of amplifier 20 (including the directcomponent applied to combiner input 91 and carrier cancellationcomponent applied to combiner input 92) is exactly the opposite of thepilot tone component injected via the directional coupler 65 andamplified by amplifier 80 and applied to the combiner input 92.

[0040] For carrier cancellation, a portion of the amplified outputsignal from the main path RF power amplifier 20 is extracted via afurther directional coupler 26 and coupled through an (output-scaling)attenuator pad 28 to input port 31 of the carrier cancellation combiner30. The second input port 32 of combiner 30 is coupled to a first output42 of a (Wilkinson) splitter 40. Wilkinson splitter 40 has its input 41coupled through a delay line 34 in a signal flow path from directionalcoupler 14. The delay imparted by the delay line 34 corresponds to thedelay in the main signal path MP through the power amplifier 20, therebyproviding time alignment between the signals applied to carriercancellation combiner 30.

[0041] The carrier components at inputs 31 and 32 of the carriercancellation combiner 30 are adjusted by the vector modulator to be ofequal carrier amplitude and anti-phase, so that carrier cancellationoccurs at output 33. By subtracting a scaled version of the amplifiedoutput from the main path amplifier 20 from the RF input signal RF_(IN)delayed through the first delay line 34, the carrier cancellationcombiner 30 provides an output (absent carrier energy) representative ofan estimate of the residual distortion produced by the main poweramplifier 20 (which includes the ‘pseudo distortion’ pilot tone producedby the pilot tone generator 24 and injected by directional coupler 22.

[0042] Carrier cancellation combiner 30 has its output 33 coupledthrough a directional coupler 35 to the vector modulator 50 installed ina distortion energy transport path feeding a first input 61 of adistortion and carrier energy combiner 60. As noted earlier, the outputof the directional coupler 35 is detected by power detector 37 andrepresents the residual carrier energy left over after carriercancellation in combiner 30. The carrier power control mechanism 116minimizes detected carrier energy, by adjusting the vector modulator 16input to the RF amplifier 20, so that its contribution at the firstinput 31 of the carrier cancellation combiner 30 is the same as that ofthe RF carrier at the second input of the combiner 30. This leaves onlythe pilot tone injected at the input of RF amplifier 20 and amplifierdistortion energy at the carrier cancellation combiner output 33.

[0043] Wilkinson splitter 40 has a second output 43 coupled to avariable gain stage 71 and variable phase shifter 73 of a vectormodulator 70 installed in an input path to a second input 62 of thepilot/distortion and carrier energy combiner 60. Vector modulator 70 isoperative to control the RF carrier energy therethrough so as to ensurethat the RF carrier energy output from amplifier 80 is the same as thatdelivered by RF amplifier 20. For this purpose, the residual power atport 94 of output combiner 90 is monitored by power detector 97 which iscoupled to input port 102 of DSP 100. The output of power detector 97 isapplied to a second carrier power minimization-based control mechanism117 within the DSP 100. This second carrier power control mechanismcontrols the vector modulator 70, so as to minimize (drive to zero) anycarrier leakage energy at the output combiner's terminated port 94, andthereby maximizing the carrier power delivered at port 93 of the outputcombiner 90.

[0044] The output 63 of the distortion and carrier energy combiner 60 iscoupled through a buffer amplifier 64 and antiphase pilot tone-injectingdirectional coupler 65 to RF amplifier 80. Thus, the input to the RFamplifier 80 includes three components. A first is RF carrier energyextracted from the RF input 10 via splitter 40 and adjusted by thevector modulator 70 to match the RF carrier energy applied to thecompanion RF amplifier 20. A second contains two distortion energycomponents—antiphase pilot tone and IMDs—which, when amplified by the RFamplifier 80, will exactly oppositely match the pilot and IMD componentsproduced at the output of RF amplifier 20.

[0045] As in the open loop architecture of FIG. 1, the output combiner90 of the multi vector modulator controlling, closed loop architectureof FIG. 2 produces a summation of the signals applied to its inputs 91and 92, so that what is produced at output port 93 is a composite RFsignal, in which the desired amplified RF carrier signals outputsproduced by the two RF amplifiers 20 and 80 sum or constructivelycombine, and unwanted distortion products (including the injected pilottones) destructively combine, or cancel.

[0046]FIG. 3 diagrammatically illustrates a second embodiment of theinvention, having feed forward loops shown in broken lines 300, that are‘wrapped around’ the closed loop ACT-based RF power amplifierlinearization architecture of FIG. 2, described above, and shown inbroken lines as ACT RF amplifier linearization stage 200. The additionof the feed forward loops 300 enables the integrated amplifierarchitecture (containing the RF amplifier linearization stage 200 andthe feed forward loops 300) to deliver extremely high linearity,including the ability to routinely achieve carrier-to-distortion ratiosas high as 85 dB. Efficiency is significantly better than that which istypically seen in competitive dual loop feed forward power amplifiers.

[0047] For this purpose, the ‘wrapped around’ feed forward loops 300 ofthe embodiment of FIG. 3 include a buffer amplifier 313 and a furtherdirectional coupler 314 coupled between the RF input terminal 10 and themain signal flow path of the ACT RF amplifier linearization stage 200. Afirst main path through the directional coupler 314 is coupled to avector modulator 316, having a processor-controlled variable gain(attenuator) stage 317 and a processor-controlled variable phaseshifting stage 318 cascaded with variable gain stage 317.

[0048] The output port 93 of the output combiner 90 of the amplifierlinearization stage 200 is coupled through a circulator 302 to adirectional coupler ³² 8, used to extract a portion of the amplifiedoutput signal from the output combiner's output port 93. This extractedoutput signal is coupled through an (output-scaling) attenuator pad 329to a first input 331 of a carrier cancellation (Wilkinson) combiner 330within a further carrier cancellation loop (loop 3). A second input 332of the carrier cancellation combiner 330 is coupled to a first delayline 336 installed in a second signal flow path from the directionalcoupler 314. The delay imparted by the delay line 336 corresponds to thedelay in the signal path through the amplifier stage 200, and serves totime-align the two signals applied to the carrier cancellation combiner330. By subtracting a scaled and antiphase version of the output ofamplifier stage 200 from the RF input signal delayed through the firstdelay line 336, the carrier cancellation combiner 330 provides a‘further refined’ estimate of the residual distortion produced by theACT RF amplifier linearization stage 200.

[0049] Carrier rejection within the feed forward stage 300 is optimizedby carrier power minimization mechanism 136 within DSP 100, which iscoupled via a DSP input port 104 to a power detector 337, coupled to adirectional coupler 338 at the output of the carrier cancellationcombiner 330. The carrier power minimization mechanism 136 executes adetected power minimization algorithm, and outputs signals forcontrolling the attenuator 317 and the phase shifter 318 of the vectormodulator 316 in the RF input path of the amplifier stage 200, so as toset the input RF carrier energy at a value that will cause the RFcarrier power in the composite output signal at combiner output port 93,when scaled by attenuator 329, to cancel the RF input energy applied tothe second input 332 of carrier cancellation combiner 330.

[0050] The distortion energy-representative output 333 of carriercancellation combiner 330 is coupled to a vector modulator 346 of afeed-forward distortion cancellation loop (loop 4). Like the vectormodulator 316, vector modulator 346 contains a processor-controlledvariable gain stage 347 cascaded with a processor-controlled variablephase shifting stage 348.

[0051] The output of the vector modulator 346 is coupled through abuffer amplifier 351 and a directional coupler 353 to a feed-forwarderror amplifier 360 of loop 4. The feed-forward error amplifier 360amplifies the gain and phased adjusted residual power amplifierdistortion derived from the carrier cancellation combiner 330 of loop 3and injects the error signal to the amplifier's output signal path viadirectional coupler 371.

[0052] The directional coupler 666 at the output of the compositeACT/Feedforward amplifier couples a portion of the distortion signal toan input port 105 of a further pilot tone receiver 667, which drives afurther pilot energy minimization mechanism 668 within DSP 100. Thisauxiliary pilot tone minimization mechanism 668 adjusts the parametersof the vector modulator 346 based upon detected pilot tone distortionenergy, to produce auxiliary distortion signal products that are equalin amplitude and opposite in phase from the original amplifierdistortion of ACT main RF amplifier stage 200. These distortion productsprovide further cancellation of any remaining distortion generated bythe main RF amplifier stage 200 when the feed forward error path isreinjected into the output of the output combiner 90 at directionalcoupler 371.

[0053] Directional coupler 371 is installed between a delay line 373 inthe output path of the output combiner 90 and the output directionalcoupler 95. Similar to the use of delay line 336, the delay imparted bythe delay line 373 equals the signal propagation delay through in erroramplifier path and serves to ensure there is adequate phase matching atthe feed forward directional coupler 371 over the operating frequencyrange. The output of directional coupler 371 corresponds to arecombination of the amplified feed-forward signal anti-phase with adelayed version of the output signal from the main path amplifier stage200, and achieves very high distortion suppression so that the linearityof the amplifier is significantly enhanced.

[0054] Directional coupler 353 samples distortion and residual pilotenergy due to imperfect adjustment of the ACT amplifier stage 200. Pilotenergy at the output of direction coupler 353 is sensed by pilotreceiver 114 and used to control ACT vector modulator 50 via a powerminimization loop 110 to minimize distortion at the output of the ACTamplifier stage 200.

[0055] In the embodiment of FIG. 2, the output direction coupler 95samples desired carrier, pilot and distortion energy at the output ofthe ACT power amplifier. In FIG. 3, the directional coupler 353 samplesthe same energy, with the carrier power substantially eliminated by thecarrier cancellation process of loop 3 of the feedforward process. As aresult, the pilot signal to carrier ratio at the coupled port ofdirectional coupler 353 is enhanced over that of the coupled port of thedirectional coupler 95 of the embodiment of FIG. 2.

[0056] For this reason, the pilot energy at the coupled port ofdirectional coupler 353 is used to control the ACT vector modulator 50via the pilot receiver 114 and energy minimization mechanism 110.

[0057] Thus, the power minimization and performance monitor controllersand associated pilot generator and pilot receiver circuits of the ACT RFamplifier linearization stage 200 maintain distortion cancellation inthe pre-distortion loop, while the feed forward loop of the feed-forwardstage 300 results in a simplification in circuit implementation thatreduces amplifier size, complexity and cost.

[0058] As will be appreciated from the foregoing description, thepotential amplifier output signal and distortion energy inequalityproblem of an open loop ACT-based RF power amplifier linearizationarchitecture is substantially minimized, by injecting a pilot tone as a‘pseudo distortion’, signal and using a set of power minimization loopsthat are closed around the RF amplifier pair. The power minimizingcontrol loops control a set of vector modulators such that both theinjected pilot tone and intermodulation distortion products arecanceled, while RF carrier components constructively sum in thecomposite output of the amplifiers. Both signal summation adjustment anddistortion cancellation adjustment may be independently maintained toprovide more than 20 dB of distortion reduction over a wide range ofsupply voltage, temperature and different input signal characteristics.

[0059] In addition, by wrapping a feed forward loop around this closedloop pre-distortion architecture, the amplitude of the distortionproducts may be decreased by up to another 35 dB. The addition of thefeed-forward loop permits an overall signal-to-distortion ratio as highas 85 dBc to be achieved. Because the closed loop ACT pre-distortionarchitecture is more efficient than a equivalent single loop feedforward power amplifier, the overall system is several percent moreefficient than a dual loop feed forward design with similar performance.

[0060] While we have shown and described a number embodiments inaccordance with the present invention, it is to be understood that thesame is not limited thereto but is susceptible to numerous changes andmodifications as are known to a person skilled in the art, and wetherefore do not wish to be limited to the details shown and describedherein, but intend to cover all such changes and modifications as areobvious to one of ordinary skill in the art.

What is claimed:
 1. An RF power amplifier apparatus comprising: an RFinput port to which an RF input signal is applied; an RF output portfrom which an amplified RF output signal is derived; first and second RFsignal processing paths coupled between said input and output ports andcontaining first and second RF power amplifiers; said first RF signalprocessing path including a first RF signal vector modulator, that isadapted to controllably adjust a first RF signal component of said RFinput signal applied to said first RF power amplifier; said second RFsignal processing path including an intermodulation distortion (IMD)extraction circuit coupled to an output of said first RF power amplifierand to said RF input port, and including a second RF signal vectormodulator adapted to controllably adjust said RF input signal, and athird RF signal vector modulator adapted to controllably adjust an IMDcomponent present in an amplified RF output signal from said first RFpower amplifier, and to generate a second RF signal component as acombination of a controllably adjusted RF input signal and saidcontrollably adjusted IMD component; an output signal combiner coupledto outputs of said first and second RF power amplifiers and to saidoutput port, and being operative to produce said RF output signal as acomposite of said first and second RF signal components, in which RFcarrier components produced by said first and second RF power amplifiersconstructively sum and IMD components produced thereby destructivelycombine; and a vector modulator controller, which is adapted to controlsaid first, second and third vector modulators independently of oneanother.
 2. The RF power amplifier apparatus according to claim 1,wherein said first and second RF power amplifiers are matched RF poweramplifiers.
 3. The RF power amplifier apparatus according to claim 1,wherein said vector modulator controller is adapted to control one ofsaid vector modulators based upon power minimization at a signal port ofsaid output signal combiner.
 4. The RF power amplifier apparatusaccording to claim 3, wherein said vector modulator controller isadapted to control said first vector modulator based upon powerminimization of carrier energy leakage in said output signal combiner.5. The RF power amplifier apparatus according to claim 1, wherein saidIMD extraction circuit includes a cancellation combiner that isoperative to differentially combine a sample of the amplified RF outputsignal produced by said first RF power amplifier with a portion of saidRF input signal to derive said IMD component, and to combine said IMDcomponent with said RF input signal applied to said second signalprocessing path, to produce said second RF signal component as acombination of said RF input signal and said IMD component, where saidIMD component is phase-inverted with respect to a carrier component atthe output of the first RF power amplifier.
 6. The RF power amplifierapparatus according to claim 5, wherein said vector modulator controlleris adapted to control said third vector modulator based upon powerminimization of monitored distortion energy the output of said carriercancellation combiner.
 7. The RF power amplifier apparatus according toclaim 1, further including a pilot tone generator, which is operative toinject a first pilot tone into the signal path through said first RFpower amplifier, and a second pilot tone into the signal path throughsaid second RF power amplifier, and wherein said vector modulatorcontroller includes a pilot tone receiver coupled to said output port ofsaid output signal combiner, and being adapted to control said thirdvector modulator based upon pilot tone energy in said output port ofsaid output signal combiner.
 8. The RF power amplifier apparatusaccording to claim 7, wherein said vector modulator controller isadapted to control said second vector modulator based upon pilot toneenergy in the composite output of said output signal combiner.
 9. The RFpower amplifier apparatus according to claim 8, wherein said secondpilot tone has the same frequency as, but complementary phase withrespect to, said first pilot tone.
 10. The RF power amplifier apparatusaccording to claim 1, further including a feed-forward RF amplifier loopcoupled to said first RF signal processing path and said output port,and being operative to provide an amplified RF distortion output signalcontaining distortion introduced by said first and second RF amplifiers,but effectively excluding said RF input signal, and wherein said outputsignal combiner is coupled to said feed-forward amplifier loop and isoperative to combine said amplified RF distortion output signal withsaid RF output signal to produce a refined RF output signal.
 11. The RFpower amplifier apparatus according to claim 10, further including afourth vector modulator coupled with said first RF signal processingpath and being adapted to controllably adjust said first RF signalcomponent of said RF input signal applied to said first RF poweramplifier, and wherein said feed-forward RF amplifier loop includes afifth vector modulator adapted to controllably adjust said amplified RFdistortion output signal, and wherein said vector modulator controlleris adapted to control said fourth and fifth vector modulatorsindependently of one another and independently of said first, second andthird vector modulators.
 12. The RF power amplifier apparatus accordingto claim 11, wherein said vector modulator controller is adapted tocontrol said fourth vector modulator based upon power minimization at asignal port of said signal combiner.
 13. The RF power amplifierapparatus according to claim 12, wherein said vector modulatorcontroller is adapted to control said fifth vector modulator based uponpilot tone energy in the composite output of said output signalcombiner.
 14. An RF power amplifier apparatus comprising: an RF inputport; an RF output port; a first RF signal processing path coupledbetween said RF input and output ports and containing a first RF poweramplifier, and a first RF signal vector modulator, that is adapted tocontrollably adjust a first RF signal component of an RF input signalcoupled to said RF input port; a second RF signal processing pathcoupled between said input and output ports and containing a second RFpower amplifier, that is effectively matched with said first RF poweramplifier, and an intermodulation distortion (IMD) extraction circuitcoupled to an output of said first RF power amplifier and to said RFinput port, said second RF signal processing path including a second RFsignal vector modulator, that is adapted to controllably adjust said RFinput signal, and a third RF signal vector modulator, that is adapted tocontrollably adjust said IMD component, and to generate a second RFsignal component as a combination of a controllably adjusted RF inputsignal and a controllably adjusted IMD component present in an amplifiedRF output signal from said first RF power amplifier; an output signalcombiner coupled to outputs of said first and second RF power amplifiersand to said output port, and being adapted to produce said RF outputsignal as a composite of said first and second RF signal components, inwhich RF carrier components produced by said first and second RF poweramplifiers constructively sum and IMD components destructively combine;a pilot tone generator, which is operative to inject a first pilot toneinto the signal path through said first RF power amplifier and a secondpilot tone into the signal path through said second RF power amplifier;and a vector modulator controller, which is adapted to control saidfirst vector modulator based upon power minimization of carrier energyin a carrier cancellation path of said intermodulation distortion (IMD)extraction circuit, said second vector modulator based upon leakageenergy in a terminated output port of said output signal combiner, andsaid third vector modulator based upon power minimization of monitoredpilot tone energy at the output of said carrier cancellation combiner.15. The RF power amplifier apparatus according to claim 14, wherein saidfirst pilot tone has the same frequency as, but complementary phase withrespect to, said second pilot tone.
 16. The RF power amplifier apparatusaccording to claim 14, further including a feed-forward RF amplifierloop coupled to said first RF signal processing path and said outputport, and being operative to provide an amplified RF distortion outputsignal containing distortion introduced by said first and second RFamplifiers, but effectively excluding said RF input signal, and whereinsaid output signal combiner is coupled to said feed-forward amplifierloop and is operative to combine said amplified RF distortion outputsignal with said RF output signal to produce a refined RF output signal.17. The RF power amplifier apparatus according to claim 16, furtherincluding a fourth vector modulator coupled with said first RF signalprocessing path and being adapted to controllably adjust said first RFsignal component of said RF input signal applied to said first RF poweramplifier, and wherein said feed-forward RF amplifier loop includes afifth vector modulator adapted to controllably adjust said amplified RFdistortion output signal, and wherein said vector modulator controlleris adapted to control said fourth and fifth vector modulatorsindependently of one another and independently of said first, second andthird vector modulators.
 18. The RF power amplifier apparatus accordingto claim 17, wherein said vector modulator controller is adapted tocontrol said fourth vector modulator based upon power minimization of asignal port of said output signal combiner, and said fifth vectormodulator based upon pilot tone energy in the composite output of saidoutput signal combiner.
 19. A method of amplifying an RF signalcomprising the steps of: (a) applying said RF signal to a first RFsignal processing path containing a first RF power amplifier; (b)controllably adjusting said RF signal applied to said first RF poweramplifier; (c) combining said RF signal and a first amplified RF outputsignal from said first RF signal amplifier in a carrier cancellationpath to produce a distortion signal containing distortion in said firstamplified RF output signal; (d) controllably adjusting said distortionsignal to produce an adjusted distortion signal; (e) controllablyadjusting said RF signal to produce a further adjusted RF signal; (f)combining said adjusted distortion signal with said further adjusted RFsignal to produce a distorted RF signal; (g) coupling said distorted RFsignal to a second signal processing path containing a second RF poweramplifier, that is effectively matched with said first RF poweramplifier; (h) combining said first amplified RF output signal from saidfirst RF power amplifier with a second amplified RF output signal fromsaid second RF power amplifier to produce at an output port an RF outputsignal in which RF carrier components produced by said first and secondRF power amplifiers constructively sum and distortion componentsdestructively combine; and (i) injecting a first pilot tone into saidfirst signal path through said first RF power amplifier and a secondpilot tone into said second signal path through said second RF poweramplifier; and wherein step (b) comprises controllably adjusting said RFsignal applied to said first RF power amplifier based upon powerminimization of carrier energy in said carrier cancellation path, step(d) comprises controllably adjusting said distortion signal based uponpower minimization of monitored pilot tone energy in said RF outputsignal produced in step (h), and step (e) comprises controllablyadjusting said RF signal to produce said further adjusted RF signalbased upon leakage energy produced in step (h).
 20. The method accordingto claim 19, further including the step (j) of coupling a feed-forwardRF amplifier loop coupled to said first RF signal processing path andsaid output port, and being operative to provide an amplified RFdistortion output signal containing distortion introduced by said firstand second RF amplifiers, but effectively excluding said RF signal, andwherein said step (h) further includes combining said amplified RFdistortion output signal with said RF output signal to produce a refinedRF output signal.